The Holy Mysteries, what the West calls the sacraments, are the appointed ways in which the grace of God is given to His people through visible, material means: water, oil, bread, wine, the laying-on of hands. The Church counts seven chief among them: Baptism, Chrismation, the Eucharist, Confession, Ordination, Marriage, and Holy Unction.
They are called Mysteries because in them something truly happens that the eye cannot see: in the water of Baptism a person dies and rises with Christ; in the bread and wine of the Eucharist the faithful receive His very Body and Blood; in Confession sins are truly forgiven. God does not save us as disembodied spirits but as whole persons, and so He comes to us through the stuff of creation, hallowing it.
The Mysteries are the pulse of the Christian life, its beginning in Baptism, its food in the Eucharist, its healing in Confession and Unction. They are not magic and not mere symbols, but the real meeting of God and His people, given to the Church and celebrated within her.